1. Field of the Invention
The present invention relates in general to a lamp driving system, and more specifically to a balance circuit for a liquid crystal display (LCD).
2. Description of the Related Art
A power supply circuit supplies an AC current to a cold cathode fluorescent lamp (CCFL) acting as a light source (or a backlight) for a LCD. A feedback and control circuit stabilizes the CCFL current. Large LCD panels usually require two or more fluorescent lamps for sufficient back lights of the LCD.
FIG. 1 illustrates a circuit diagram of a conventional lamp driving system. The lamp driving system comprises a driving circuit 10, a transformer 12, and a feedback and control circuit 20, for driving a first lamp 62 and second lamp 64. The first current I1 and second current I2 of the first lamp 62 and second lamp 64 are not balanced, due to different impedances of the first lamp 62 and second lamp 64. The lamp driving system, shown in FIG. 1, controls the total current of the first lamp 62 and second lamp 64 but can not balance individual currents through the first lamp 62 and second lamp 64. The imbalance not only reduces luminance, but also shortens the lifetime of the lamps.
FIG. 2 illustrates a circuit diagram of another conventional lamp driving system for driving a first lamp 62, a second lamp 64, a third lamp 66, to nth lamp 68. In the lamp driving system, a driving circuit 10 is used to convert a DC signal to an AC signal. In addition, a transformer 12 has a primary winding 12a coupled to the driving circuit 10, and a secondary winding 12b outputting the AC signal. A balance circuit 30, coupled to these lamps 62˜68, balances the currents from the 10 first lamp 62, the second lamp 64, the third lamp 66 . . . to the nth lamp 68. The balance circuit 30 is a multi-winding transformer with every winding distribution coupled to a lamp, and equal number of windings wound on the same magnetic core. The balance circuit 30 can be coupled to the high voltage terminals or low voltage terminals of the lamps. A feedback and control circuit 20 is coupled to the first lamp 62, the second lamp 64, the third lamp 66 . . . to the nth lamp 68 so as to control the driving circuit 10 according to the first current I1, the 20 second current 12, and the third current I3 . . . to the nth current In.
FIG. 3 shows root-mean-square current (Irms) waveforms of the first lamp and second lamp in FIG. 2, FIG. 4 shows sine-wave current waveforms of the first lamp 25 and second lamp in FIG. 2. The balance circuit 30 of FIG. 2 can not sufficiently control the error values of first current 62 and second current 64 to balance lamp currents.
In this lamp driving system, the balance circuit 30 30 is a transformer with multiple windings and each of the windings has the same winding number To effectively control current errors produced by first lamp 62 and second lamp 64, the inductance value of every winding of the transformer (balance circuit 30) must be very high (greater than 1 Henry), so as to achieve current balance. FIG. 5 is a root-mean-square current (Irms) waveform showing a first and second lamp controlled by a current balancing circuit using an inductance of two Henry, and FIG. 6 is a current sine-waveform diagram thereof. As shown in FIGS. 5 and 6, the first current I1, the second current I2, the third current I3, . . . to the nth current In can achieve a better balance by increasing inductance value of the balance circuit 30 (transformer with multiple windings). However, as inductance of transformer is increased, such as greater than one Henry, the volume of the transformer is also increased, and then the manufacturing process becomes complicated and the manufacturing costs is further increased.